1. Field of the Invention
The invention relates to an optical disc drive, and more particularly, to a method of determining the size of an optical disc inserted into optical disc drive.
2. Description of the Prior Art
In typical optical disc drives, users can insert media having different sizes. The compact disc (CD) standard specifies two sizes: 8 cm and 12 cm in diameter. Apart from being able to report the size of the inserted disc to the user, this information might also be necessary for the Turn-Table-Motor (TTM) controller that drives and controls the speed of the disc. Since the moment of inertia of an 8 cm disc and a 12 cm disc differ greatly, a different TTM controller might be required for rotating each type of disc in order to maintain fast, stable, and robust control over the rotation speed of the disc.
FIG. 1 is a partially cut-off perspective view of an optical disc drive of the prior art. In FIG. 1, an optical disc 1 is placed on a turntable 2 and is rotated by a turntable motor 3. Opposed to the lower face of the optical disc 1 there is provided an optical pickup 4 which is supported by a guide rail 5 and a driving screw 6. The optical pickup 6 moves in the radial direction of the optical disc 1 by rotation of the driving screw with a stepping motor 7.
In a typical optical disc drive, the decision of determining whether an 8 cm disc or a 12 cm disc is present in the drive is made in one of two ways. The size can be determined mechanically using switches in the disc-loader module. This method involves additional switches in the loader system, and therefore increases the bill of materials of the optical disc drive. On the other hand, the size can also be determined at start-up using the so-called kick phase. This method is a software solution for detecting the disc size, and therefore potentially does not affect the bill of materials of the optical disc drive.
For the software method, in the kick phase a constant voltage is applied to the TTM that drives the disc in order to accelerate the disc quickly to the target velocity where the TTM takes over velocity control of the TTM. The duration of this kick phase, i.e. from the start of the applied constant voltage until the TTM controller takes over, is a measure of the mass of inertia of the load driven by the TTM and therefore a measure of the disc size.
The disadvantage of the software method of the prior art is that the disc speed needs to be measured during the complete kick phase, or at least at the very end of the kick phase. Although the speed signal might be available for certain TTM types, e.g. for brushless TTM types where in most cases the motor electronics provides such a signal, this is not the case for brush TTM motor types. Here extra effort, and hence cost, must be made to extract this kind of information. The TTM velocity control after the kick phase on these brush TTM drives is based on the data rate of the information stored on the disc. This data rate speed signal, which in a CD module or a digital versatile disc (DVD) module is usually provided by the data acquisition phase locked loop (PLL), becomes available at a later stage of the start-up phase (after several other control loops have been closed and data can be read) and can therefore not be used to determine the disc speed at the end of the kick phase.
Another disadvantage of deciding between 8 and 12 cm discs during the kick phase is that this only works when the initial disc speed equals zero at the start of the kick phase. This means the disc must be slowed down to a standstill before the kick phase starts. Otherwise the detection might provide incorrect results, such as a 12 cm disc being detected as an 8 cm disc, which might be catastrophic if the TTM controller depends on a correct detection of the disc size.